[LyX/master] Fix nullptr warnings

[Richard Kimberly Heck]

commit 12d164393e4dd0cc60b48be57e5abf1ba43b23b3
Author: Richard Kimberly Heck <rikiheck at lyx.org>
Date:   Fri Feb 28 01:42:32 2020 -0500

    Fix nullptr warnings
---
 src/ConverterCache.cpp |    6 +++---
 1 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/src/ConverterCache.cpp b/src/ConverterCache.cpp
index 2373b84..c601c3f 100644
--- a/src/ConverterCache.cpp
+++ b/src/ConverterCache.cpp
@@ -198,14 +198,14 @@ CacheItem * ConverterCache::Impl::find(FileName const & from,
 		string const & format)
 {
 	if (!lyxrc.use_converter_cache)
-		return 0;
+		return nullptr;
 	CacheType::iterator const it1 = cache.find(from);
 	if (it1 == cache.end())
-		return 0;
+		return nullptr;
 	FormatCacheType & format_cache = it1->second.cache;
 	FormatCacheType::iterator const it2 = format_cache.find(format);
 	if (it2 == format_cache.end())
-		return 0;
+		return nullptr;
 	return &(it2->second);
 }